This invention relates to thermoplastic films which are capable of receiving ink derived from liquid toner, especially liquid toner employed in electrostatic printing.
Electrostatic printing is a very effective method of image transfer commonly used in photocopying and photo printing. Typically, in electrostatic printing, a potential electrostatic image is formed on an imaging surface carrying a uniform electrostatic charge. The uniform electrostatic charge can be created by exposing the surface to corona discharge. The uniform electrostatic charge is then selectively discharged by exposing it to a modulated beam of light which corresponds to an image formed from an original. The discharged surfaces form the background while the charged surfaces form the print image. The print image is developed by applying pigmented toner particles which adhere to the charged "print" portions of the surface. The pigment is subsequently transferred by various techniques to a copy sheet.
Dry toner is most commonly used in electrostatic printing. The quality and clarity of the image and image resolution, is related to the size of the toner particles. While it is thought that very fine particles will produce a finer image, there is a practical limitation on the size of toner particles which can be used. Dry toner particles must be of sufficient weight and size to be deposited onto the print surface without becoming airborne, which is thought to lead to machinery fouling and, possibly, environmental problems. Additionally, in fixing the image, the dry toner particles are fused onto the paper by exposure to very high temperatures, e.g. in excess of about 400.degree. F. (204.degree. C.). This energy requirement is a significant drawback.
Paper is widely used as the image receiving element in electrostatic imaging. It would be advantageous to use plastic as the receiving element. Among other advantages over paper, plastic is moisture resistant, flexible and heat sealable and plastic substrates can be either clear or opaque. However, the high temperatures necessary for imaging with the dry toners will melt plastic films and the liquid toners do not transfer well and adhere to uncoated plastic.
Plastic films which can receive printing inks have been disclosed; however, these films require specialized treatment to enable them to receive the dry toner. For example, a layer of plastic film used as a laminate for corrugated paperboards is disclosed in U.S. Pat. No. 4,871,406. The film is a thermoplastic co-extruded polymeric film, such as, polypropylene with ethylene acrylic acid. This film can receive printing inks only after being treated with a corona discharge device or a high velocity flame. Another example is U.S. Pat. No. 4,853,290 which discloses laminates of polypropylene film prepared by coextruding a polymer composition onto a polypropylene film. This polypropylene film requires corona treatment after printing on its surface.
Corona and flame treatments oxidize a plastic surface in order to enable the fixing of dry printing inks. Besides adding time, money and inconvenience to the production process, these treatments pose safety and environmental hazards. A corona treatment process generates toxic ozone so the atmosphere in the treating step must be contained and exhausted from the operating area. At high film speeds, a layer of air that contains ozone adheres to the film as it leaves the treating enclosure. This layer must be removed from the film by close-fitting baffles or a vacuum system. In some cases corona treatment requires special chambers and needs to be carried out off-line. Flame treatment entails impinging a flame directly onto a film surface as the film is moved across a cooling roll. The complexity of controlling the flame-treating process presents safety hazards.
To overcome these disadvantages, liquid toners have been developed in which the toner is dispersed in a solvent. The solvent is removed in the last printing step by the mechanism of the press. Because of the liquid medium, very fine dye particles can be employed without concern for the particles becoming airborne. Thus, copies of very high resolution can be made and high temperatures needed to fuse dry toners are not required. Liquid toners for electrostatic imaging are described in U.S. Pat. Nos. 5,225,306; 5,276,492; 5,346,769 and 5,407,771.
In order to enable plastic films to be printed upon with liquid toner, coatings can be applied which are able to receive inks (see U.S. Pat. No. 5,215,817). However, the use of coatings presents difficulties. The coating process adds time, money and inconvenience to the production process, and presents practical limitations.
Typical systems used for coating plastic films are the dispersion, solvent and extrusion coating processes.
The dispersion coating system is a multi-step, complex process that requires vigilant monitoring.
The dispersion coating process involves unwinding the film, applying the coating uniformly at the desired thickness, waiting for the coating to dry and rewinding the film into a uniform roll. Coating thickness should be measured across the film as part of the coating sequence. This can be done using radiation absorption. As the chemical nature of the coating more nearly approaches the substrate, this measurement becomes increasingly difficult.
The conditions of the dispersion coating process must be carefully monitored in order to ensure adequate coalescence of the coating. The dispersion coating polymer is dispersed in water often with a surfactant. Once the coating is applied, the water is evaporated. In order for a film to form, conditions need to be highly regulated. If evaporation occurs at room temperature, the dried polymer usually forms a fragile, uncoalesced coating. The particles need to make intimate contact with each other in order for coalescence to occur. Then diffusion and interpenetration of the polymer molecules must occur readily across the particle interfaces. The effectiveness of this diffusion depends on the mobility of the polymer molecules which in turn depends on the temperature of and the viscosity of the liquefying particle, which is a function of molecular weight. Therefore, a balance must be reached between a molecular weight low enough for coalescence but high enough for adequate toughness and flexibility of the coating film. Moreover, the surfactant used to create a stable dispersion can act as a barrier to interpenetration at the surface of the particles. Therefore an optimum must be found in the concentration of the surfactant.
The solvent coating system also entails a multi-step process. This process involves unwinding the film, applying the coating uniformly at the desired thickness, waiting for the coating to dry and rewinding the film into a uniform roll. The desired film thickness can be achieved by the use of radiation absorption.
An example of a solvent coating process for plastic films, which provides a surface that will receive liquid toners, is a polyamide solution sold under the name TOPAZ by Indigo Company.
In the solvent coating process, the coating polymer is dissolved in a solvent. The solvent is evaporated once the coating is applied leaving behind a coating film. However, as the coating develops it acts as a solvent barrier which makes it difficult to drive off the last traces of residual solvent. An additional disadvantage in the solvent coating process is that the solution is sensitive to ambient conditions. The solution is difficult to handle at low temperatures (it tends to lose solubility) and the coating absorbs atmospheric moisture which may make the film tacky even after drying. Among others, this can pose blocking problems.
Significantly, the polyamides used in the solvent coating process can present environmental hazards. Disposal of the solvent must be conducted in a proper manner. Moreover, recovery of solvent, which has become airborne, is complex.
In the extrusion coating process, the substrate film must be unwound, fed through a quench roll or between a quench roll and a nip roll to receive the falling polymer melt, and then the film must be rewound again onto a roll.
Additionally, practical considerations narrow down the field where the extrusion coating process can be utilized. Given the high temperature of the polymer melt, the substrate film must have high thermal stability. Additionally, there is a limitation on what can be used as a melt coating polymer. Very high melting polymers, polymers with low melt strength, and heat sensitive polymers are ruled out. Furthermore, coating thickness is limited at the low end to about 0.2 mil under the best conditions. (More comprehensively reviewed in Plastic Films by K. R. Osborn and W. A. Jenkins 1992, incorporated herein by reference.)
Thus, it is an object of the present invention to overcome difficulties presently encountered in the art by providing a coextruded thermoplastic film which is capable of receiving ink derived from liquid toner without the need of, among other things, coating(s) and/or post-extrusion or post-printing processes.